The subunit composition of the mammalian Mediator complex is not conserved in all vertebrates: Insights from evolutionary plasticity of fish genomes

The Mediator complex is an indispensable, multi-subunit protein transcriptional coactivator with a central role in gene expression in Eukaryotes. Among vertebrates, its molecular structure and subunit composition are well-known only for mammals. Genes encoding fish Mediator subunits remain unknown even for zebrafish that is otherwise the best explored fish species. Here, we first reconstructed genes encoding Mediator subunits from 12 brain transcriptomes of a percid fish, pikeperch (Sander lucioperca). These data revealed two additional fish-specific paralogous genes (med13b and med31l) and a missing paralog (med12l) in comparison to genes of mammalian Mediator subunits. The Med13 subunit is encoded by two paralogs in basal (coelacanth) as well as derived sarcopterygians (mammals), while three paralogs are present in numerous but not all fish lineages. All three med13 paralogs were highly transcribed in the juvenile pikeperch brain. Our molecular-phylogenetic analysis of the three fish med13 paralogs revealed the evolutionary origin of the additional med13b paralog from the teleost-specific genome duplication. The additional paralog med31l encoding the Med31 subunit shows a more limited occurrence among fishes with potentially different ways and times of its origin-tandem duplication on the same chromosome, translocation in the opposite strand of another chromosome or a consequence of whole-genome duplication. The mammalian Mediator complex is not universal to all vertebrates and paralogs encoding subunits of the Mediator complex are not conserved across all vertebrates. Further research is needed to explore fish-specific genes encoding subunits of the Mediator complex and their tissue-specific transcription.